If I'm not mistaking Magento tries to make a smart estimate on what the interval in the rate should be. For example, if you have a category with products that have a range between $0 and $100, the range slide will be in steps of $10.
But, if you have a category that has products varying from $0 to $10000, the range will probably be in steps of $1000.
A side-effect (feature) if this behaviour is if you have a situation where one product in your category is much more expensive than the rest. For example: let's say you have a category with 1000 products. 999 of those products are in the range of $0 to $100, but 1 product costs $9000. Magento will treat the category has having products in the range $0-$9000 and probably show intervals of $1000.
I'm not sure where in the part of the code you can alter this behaviour, or even if it's a configuration setting. The fact that you're running a very old version of Magento also isn't very helping.
I hope this information helps you somewhat.